Introduction to Epithelium (Mhawi) PDF

(Introduction to Epithelium) Epithelium 1 Amir Mhawi, DVM, PhD Ross University School of Medicine [email protected] Introduction This lecture describes the various general and specific characteristic features of the various epithelia of the body. The students will appreciate the structures and fu...

(Introduction to Epithelium) Epithelium 1 Amir Mhawi, DVM, PhD Ross University School of Medicine [email protected] Introduction This lecture describes the various general and specific characteristic features of the various epithelia of the body. The students will appreciate the structures and functions of the various modifications of the apical, basal, and lateral surfaces of the epithelial cells. These modifications contribute significantly to the health and well-being of the individual. The lecture briefly highlights how the epithelial cells attach to and communicate with each other via the different types of cell-to-cell junctions or cell-to-matrix junctions. Learning Objectives • Describe the various general and specific characteristic features of the various epithelia of the body. • Identify and compare the functions of 8 or more morphologically different types of epithelium and where they are located in the body. • Describe the structures and functions of the various modifications of the apical, basal, and lateral surfaces of the epithelial cells. • Identify the different types of cell-to-cell and cell-to-matrix junctions. • Explain how the many functions of epithelia, including protective, secretory, excretory, lubrication, absorptive and reproductive, contribute to the health and well-being of the individual. Epithelial Tissue (Epithelium) • Epithelium is one of four basic tissues. • The other three basic tissues are connective, nervous, muscular. • Epithelium is avascular tissue. • Depends on diffusion of nutrients from blood vessels in the underlying tissue. • Comprised mainly of packed cells whose shape and arrangement correlate with their function. Epithelium lining intestine Epithelium covering fingertips Epithelium • Cell characteristics: • Arranged as sheets. • Covers external surfaces e.g., skin. • Lines internal surfaces e.g., intestine and kidney tubules. • Or as masses (in glands). • Placed very close to each other (very limited intercellular space). • Possess intercellular junctions. • Cells are polarized. • Display distinct surface domains. • Apical (free), lateral, and basal domains. • Properties of each domain are determined by presence of specific membrane proteins. • Rest on an underlying basal lamina. • Non-cellular structure • Protein-polysaccharide-rich layer Epithelium-Classification • Classified by: • Cell arrangement (simple or stratified) • And cell shape (squamous, cuboidal, or columnar) • Usually by a combination of both arrangement and shape • Not by function Epithelium-Classification cont. • Squamous • width of cell is greater than height • Simple Squamous • One cell layer • Location: • Lines blood and lymphatic vessels. • Wall of Bowman’s capsule in kidney. • Covers mesentery. • Lines respiratory spaces (alveoli) in lung. Simple squamous epithelium lining vein (arrowheads). Arrows point to smooth muscle cells in the wall of the vein. Simple squamous epithelium (arrows) in the wall of the Bowman’s capsule of the kidney. Epithelium-Classification cont. • Special terminologies for the simple squamous: • Endothelium (upper image) Endothelial cells • Name given to simple squamous epithelia lining blood and lymph vessels and the ventricles and atria of the heart. • Mesothelium (lower image) • Name for simple squamous epithelia that line walls and cover contents of closed cavities of body: • Abdominal cavity • Thoracic cavity Simple squamous epithelium covering the peritoneum (mesothelium; arrows). Some blood capillaries are indicated by arrowhead. PT (pararosanalinetoluidine) stain. Epithelium-Classification cont. • Cuboidal • Width, depth and height approximately the same. • Simple Cuboidal • One cell layer • Found in: • Wall of thyroid follicles • Walls of kidney tubules • Surface of ovary (germinal epithelium) Walls of thyroid gland is made of simple cuboidal epithelium. Arrows point to capillaries. Simple cuboidal epithelium from kidney collecting ducts (upper part of the ducts). ovary Epithelium-Classification cont. • Columnar • Height of cell greatly exceeds width. • Simple Columnar • One cell layer. • Lines: • Intestinal tract from stomach to rectum • Uterus and cervix • Kidney collecting ducts (lower part) Small intestine lumen is lined by simple columnar epithelium. Simple columnar epithelium covers the inner cavity of the uterus. Simple columnar epithelium from the kidney collecting ducts (lower parts of the ducts). Epithelium-Classification cont. • Stratified epithelium • Two or more cells thick • Types • Stratified squamous • Keratinized and non-keratinized • Stratified cuboidal • Stratified columnar • Transitional • Stratified Squamous • Multiple layered. • Superficial layer is squamous. • Functions as barrier. • Protection of body. • Can be keratinized (skin). Skin. Stratified squamous epithelium keratinized At puberty, the rising levels of estrogen promote the maturation, proliferation, and accumulation of glycogen in the vaginal epithelial cells. Glycogen is catabolized to lactic acid, which along with components of the epithelial mucosal barrier, provide an effective first-line defense against invading pathogens. • Location: • • • • • Esophagus. Stratified squamous epithelium non-keratinized Epidermis Lining of oral cavity Lips Lining of esophagus Lining of vagina Vagina. Stratified squamous non-keratinized Note that the cytoplasm of most of the epithelial cells of the vagina looks empty due to the washing out of the glycogen during the preparation of the tissue for examination. Epithelium-Classification cont. • Stratified Cuboidal • More than one layer of cuboidal cells • Located in: • Ducts of sweat gland • Larger ducts of exocrine glands (mammary gland, as an example) • Anal canal (most distal portion of the GIT) • Function as: • Barrier • Conduit Duct (solid arrows) of sweat gland lined by stratified cuboidal epithelium. The stratification (represented by two rows of nuclei) is visible in the oblique section of the duct (red star). Epithelium-Classification cont. • Stratified Columnar • More than one layer of columnar cells • Located in: • Largest ducts of exocrine glands • Anal canal • Functions as: duct lumen • Barrier • Conduit Wall of the excretory duct of the salivary gland is comprised of stratified columnar epithelium. Epithelium-Classification cont. • Transitional (AKA urothelium) • Stratified. • Upper most cells are dome-shaped (arrows in the lower image). • Functionally accommodates distension. • Serves as barrier. • Located in: • • • • Ureters Urinary bladder Renal calyces Urethra Transitional epithelium lining the ureter Transitional epithelium. Urinary bladder. Note dome-shaped surface cells (arrows) Epithelium-Classification cont. • Special Classifications: • Pseudostratified • • • • • Has appearance of being stratified. Some cells do not reach the free surface. ALL cells rest on the basal lamina. It is actually a simple epithelium. Limited distribution in body: • Upper respiratory tract (trachea, bronchi) • Epididymis (male reproductive system) • Ductus deferens (male reproductive system) BM, basal lamina; LCT, loose connective tissue Pseudostratified columnar ciliated epithelium. Green arrows point to mucus-secreting (goblet) cells. Black arrows indicate the cilia. Pseudostratified columnar epithelium lining the duct of the epididymis of the male reproductive system. The lumen of the epididymis contains sperm. The appendages projecting from the free cell surface into the lumen are not to be confused with cilia. They are stereocilia (extra-long microvilli). Basal Lamina • AKA Basement Membrane • Acellular structure • Structural attachment site • Attaches the overlying epithelial cells to the underlying connective tissue. • Its components are synthesized and secreted by epithelial cells • Components assembled extracellularly at the base of the epithelial cell. • Basal lamina can be demonstrated with: • Periodic acid-Schiff stain (PAS) • Silver salts Basal Lamina cont. • After conventional TEM observation, basal lamina reveals two layers: - Lamina densa - Contains network fine filaments. - Lamina lucida (lamina rara) - Clear space between base of cell and lamina densa. - Believed to be an artifact caused by the shrinkage of the epithelial cells. Lamina densa Lamina lucida Basal Lamina-Function • Structural attachment • Epithelial cells to connective tissue • Compartmentalization • Separates connective tissue from epithelia, nerve, muscle tissue. • Connective tissue and its specializations are all within a continuous single compartment. • Molecules require to move from one tissue to another (from one compartment to another). Must cross basal lamina. Basal Lamina-Function cont. • Filtration • Movement of blood filtrate in the kidney • Is regulated by basal lamina • Through negatively charged molecules in lamina rara and network of collagen fibrils in lamina densa • i.e., filtration is regulated by ion exchange and molecular sieve. Basal Lamina-Composition • Basal lamina composed of different molecules. • Molecules are synthesized by the epithelial cell. • And self-assemble into basal lamia sheet extracellularly. Basal Lamina-Composition cont. • Laminins • • • • Glycoprotein molecules. Polymerize at the basal cell surface. Possess binding sites for integrins. Therefore, link basal lamina to basal plasma membrane. laminins Basal Lamina-Composition cont. • Collagens • • • • Mainly type IV collagen. Consists of short filaments. Provides structural integrity to basal lamina. Serve as molecular sieve. collagen Basal Lamina-Composition cont. • Entactin/Nidogen • Glycoprotein • Serves as a link between laminins and type IV collagen network. • Support cell adhesion. Basal Lamina-Composition cont. • Proteoglycans • Make up bulk volume of basal lamia. • Consist of protein core to which negatively charged. Glycosaminoglycans (GAGs) are attached. • Extensively hydrated. • Play a role in regulation of ions passage across the basal lamina. Chondroitin sulfate (GAG) keratan sulfate (GAG) External Lamina • Peripheral extracellular electron-dense material • Visible in TEM. • Appears the same as basal lamina. • Positive staining with PAS and sliver. • Found on external surface of nonepithelial cells: Sch Sch A • Muscle cells • Nerve-supporting cells (Schwann cells) • Adipocytes (fat-storing cells) TEM of myelinated axon (A) in cross section. Schwann cell (Sch), responsible for the myelination of the axon, is surrounded by external lamina which appears as a moderately dense ribbon at the periphery of the cell (red arrows). Myelin sheath is visible as many concentric layers of electron dense material (stars). Cell Surface Modifications • Surface specializations relate to special functions: • Microvilli • Cytoplasmic processes extending from cell surface • Stereocilia • Microvilli of unusual length • Cilia • Motile and non-motile cytoplasmic processes • Lateral and basal cell surface folds and processes • Invaginations and evaginations of cell surface • Create interdigitating and interleaving tongue and groove margins • For apposed cells Cell Surface Modifications cont. Microvilli • Distinctive cytoplasmic extensions at apical surface seen with LM in fluid transporting cells of gut and kidney. • Provide enormous increase in free surface area. • Number and shape of microvilli • Correlate with absorptive capacity. In the small intestine called striated border. In the kidney tubules called brush border. Microvilli cont. • Contain conspicuous core of Actin microfilaments. - Actin microfilaments are anchored to plasma membrane at tip and sides. - Extend down where they interact with horizontal network of actin filaments, the terminal web. - Filaments of terminal web are contractile. - This could result in decreasing diameter of apex of intestinal absorptive cell. Longitudinal section - Thereby the microvilli spread apart and the intermicrovillus space increases. - Leads to an increment in the effective absorptive surface between microvilli. Cross section Cell Surface Modifications cont. Stereocilia (stereovilli) • Extremely long immotile processes extending from apical cell surface. • Like microvilli, they are supported by actin filaments. • Found in: • Epididymis • Facilitate absorption of fluid. • Aggregate into pointed bundles • (like wet paintbrush). Cell Surface Modifications cont. Lateral folds • Lateral surface shows tortuous boundary. • Due to infoldings of cells with neighbors • Increase lateral surface. • Prominent in cells that transport fluid rapidly. • e.g. intestinal epithelium • Water in intestine enters cells apically. • Then leaves cells at lateral surface. • Water osmotically follows sodium ions that are actively transported across the lateral plasma membrane. Lateral folds basement membrane basement membrane Cell-to-Cell Adhesion/Junction cont. • Junctional Complex consists of • • • • Zonula occludens (AKA tight junction) Zonula adherens Macula adherens (AKA DESMOSOME) Gap junctions Cell-to-Cell Adhesion/Junction cont. • Zonula occludens (AKA tight junction) • Outer leaflets of adjoined plasma membranes are fused. • Occurs in band or belt-like configuration around cell. • Seal created by transmembrane proteins (occludins and claudins). • That traverse the outer leaflets of the cells. • And join occludins (or claudins) of the neighboring cell in the intercellular space. • Cytoplasmic domain of occludins/claudins attach to actin filaments. • Functions: • Control the free molecular movement between the cells. • Prevent diffusion of membrane proteins between the apical and lateral membrane domains. Actin filament blood capillary lumen endothelial cell endothelial cell membrane fusion membrane fusion connective tissue Tight junction between endothelial cells Cell-to-Cell Adhesion/Junction cont. • Zonula adherens (AKA adhering junction) • Provides lateral adhesion between epithelial cells. • Adherens occurs in band or belt-like configuration around cell. • Formed by the binding of transmembrane proteins cadherin of neighboring cells. • Cytoplasmic domain of cadherin is linked to the actin filaments of the cytoskeleton. • Morphology and integrity of adherens are calcium dependent. • Removal of Ca++ disrupts this junction. • Characterized by uniform 15-20 nm space between apposed membranes. Occludin transmembrane protein Claudin transmembrane protein Cadherin transmembrane protein Actin filament • Fascia adherens • Sheet-like junction • Called fascia because it has a broad face. • Found in intercalated disc of cardiac muscle. Cell-to-Cell Adhesion/Junction cont. • Macula adherens (desmosomes) • • • • Small localized on the lateral domain of the cell Strong attachment structure Transmembrane proteins from neighboring cells interdigitate with each other. In simple cuboidal or simple columnar epithelium found in conjunction with occluding and adhering junctions (slides 35 and 37). • TEM shows a very dense material on cytoplasmic sides of the membranes of adjoining cells. • Called attachment plaque. • Attaches to intermediate filaments. • Intermediate filaments appear to loop through attachment plaques. • And extend back out into cytoplasm. • Role in dissipating physical forces throughout the cell from attachment site. Cell-to-Cell Adhesion/Junction cont. • Gap junctions or Nexuses • Found in: • • • • Epithelia Smooth muscle cardiac muscle Nerves • Needed where activity of adjacent cells must be coordinated: • Heart muscle • Consist of accumulation of Transmembrane channels (connexons) • Each connexon comprised of six connexin proteins. • Connexons in one cell membrane precisely aligned with corresponding connexons on membrane of adjacent cell. • Tightly packed connexons create gap junctional plaque (upper right image). • Allow: Gap junction plaque as it appears in TEM freeze-fracture technique. Each little dote within the plaque is a connoxon. • Communication • Exchange of: - Ions - Regulatory molecules - Small metabolites Gap junction plaque (between the arrows) as it appears in TEM thin plastic section. Cell-to-Matrix Adhesion • Hemidesmosomes • Half-desmosome located on basal plasma membrane of the cell. • In stratified squamous epithelium connect the basal plasma membrane of the cells to basal lamina (illustration). • Found in epithelia subject to abrasion and mechanical shearing. • Skin • Cornea • Mucosae of: • Oral cavity • Esophagus • Vagina Illustration of the keratinized stratified squamous epithelium of the skin = Hemidesmosome = Desmosome = Basal lamina Hemidesmosomes cont. • TEM exhibits intracellular attachment plaque. • On the cytoplasmic side of the basal plasma membrane. • Composed of plectin and BP 230 proteins. • Intermediate filaments of the cytoskeleton bind to the proteins of the attachment plaque. • Transmembrane proteins integrins bind the attachment plaque to the extracellular matrix. Cell-to-Matrix Adhesion cont. • Focal adhesions • Dynamic attachments • Linking actin filaments to extracellular matrix proteins. • Role in attachment and migration of cells. • Focal adhesion consists of: • Cytoplasmic part • Actin filaments • Transmembrane region • Integrins • Extracellular part • Laminin and fibronectin The End of the Lecture! Extra reading if desired: Junquiera’s Basic Histology Relevant portions of Chapter 4, “Epithelial Tissue” Digital Histology https://digitalhistology.org/